a) Field of the Invention
The present invention relates to a method of manufacturing an electric field emission (electron emission by electric field) type device.
b) Description of the Related Art
A vacuum microelectronic device technique has recently become remarkable. This technique utilizes a fine processing technique of semiconductor integrated circuits to form a minute cold cathode electron source which is used for ultra fine amplifier devices, integrated circuits, flat display units, and the like. To realize practically usable vacuum microelectronic devices, it is essential to develop a cold cathode electron source capable of reliably flowing a large current upon application of a low voltage. The cold cathode electron source is mainly classified into an electric field emission type that electrons are emitted from a sharp tip of an emitter electrode by a concentrated electric field, and another type that high energy electrons are generated in semiconductor by means of avalanche effects or the like and emitted to the outside of the semiconductor. The emitter electrode is classified into a vertical emitter having a sharp needle tip formed on a substrate in the vertical direction and a lateral emitter having a sharp needle tip formed on a substrate along the substrate surface.
A method of manufacturing an electric field emission type electron source of a lateral emitter type has been proposed (refer to S. Zimmerman, Abs. 3rd Int. Vacuum Microelectronics Conf., Monterey, 1990, 1-4). With this method, as shown in FIG. 43A, a recess 102 having a vertical side wall is formed in a substrate 101. A sacrificial layer 103 is deposited by direction-less (isotropic) conformal deposition and thereafter an electron emitting material layer 104 is deposited as shown in FIG. 43B, and finally an emitter 104a is formed by removing the substrate 101 and sacrificial layer 103 as shown in FIG. 43C.
Conformal deposition forms a film having the same thickness both on the horizontal and vertical surfaces. The recess is completely filled with the film when the thickness of the film on the vertical surface of the recess exceeds a half of the width of the recess. A cusp of an inverted cone shape is formed on the surface of the film above the recess. The depth of the cusp is less than the thickness of the film.
With the above method, in order to obtain an emitter mold with a cusp of an inverted cone shape having a desired depth, it is necessary to deposit the sacrificial film thicker than the desired depth of the cusp. However, if a thick sacrificial layer is deposited by a single process, cracks are likely to be formed by thermal stress generated when the layer is cooled after the deposition. If the cracks are generated in the electron emitting material, an emitter having a desired shape cannot be obtained so that an electric field emission type device having a desired performance cannot be obtained.
With this method illustrated in FIGS. 43A to 43C, the sacrificial layer is formed by deposition conformal to the surface of the recess with a vertical side wall, i.e., deposition having good step coverage. With this conformal deposition, the radius of curvature of the bottom edge of the cusp A formed on the sacrificial film 103 is likely to become large in the order of 50 nm as shown in FIG. 44A, and it is difficult to form an emitter having a sharp tip.
If deposition having poor step coverage is used, the thickness of the film on the vertical surface is less than that on the horizontal surface. Even if a sacrificial film having the same thickness as that shown in FIG. 44A, the recess is not completely filled with the film and overhangs 105 are formed as shown in FIG. 44B. It is therefore impossible to form an emitter mold having a cusp of an inverted cone shape. Even with this method, if the sacrificial film 103 is made thicker, the overhangs contact together as shown in FIG. 44C, and it might be possible to form an emitter mold having a cusp of an inverted cone shape. However, in this case, it is difficult to obtain a small apex angle of the cusp. Furthermore, the sacrificial film is made thicker than the depth of the emitter mold so that cracks are more likely to be formed.
Another method of manufacturing a vertical type emitter has been proposed as disclosed, for example, in Japanese Patent Laid-open Publications Nos.4-61729 and 5-225895. With this method, on a substrate 106 having a predetermined crystal plane such as (1 0 0), an etching mask 107 is formed as shown in FIG. 45A. The substrate 106 is anisotropically etched to form a pyramid recess 108 having the (1 1 1) plane or the like as shown in FIG. 45B. An electron emitting material layer 109 is deposited as shown in FIG. 45C, and an emitter 109a is formed by removing unnecessary regions as shown in FIG. 45D.
With this method, the recess is pyramid-shaped and its apex angle is determined by the angle of the crystallographic plane of the substrate. If the recess formed by anisotropic etching is used for forming an emitter mold, it is difficult to obtain an emitter having a tip of a small apex angle. The emitter tip of a pyramid shape does not show stable current emission characteristics. As substrates capable of being anisotropically etched, single crystal silicon, GaAs, and the like having the (1 0 0) plane are utilized, however the etching is limited to wet etching. The degree of design freedom is limited and fine processing of device is difficult.
Another method using anisotropic etching has been proposed as disclosed in Japanese Patent Laid-open Publication No.5-172703. As shown in FIG. 46A, this method uses a structure that a silicon substrate 106 and a silicon layer 111 are laminated with a silicon oxide film 110 being interposed therebetween. An etching mask 112 is formed on the silicon layer 111, and anisotropic etching is performed. Thereafter, the etching mask 112 is removed and as shown in FIG. 46B an oxide film 113 is formed by heat treatment. The oxide film 113 forms on its surface a cusp because of its volume expansion. An electron emitting material layer 114 is deposited on the oxide film 113.
With this method, although the apex angle of the cusp can be made small by oxidizing a recess, it is difficult to obtain a cusp having a small apex angle before the oxidation treatment. Substrates to be used are limited, the degree of design freedom is small, and fine processing of device is difficult.